The role of accessory cells in the immune response is only partially understood. To date it has been established that an accessory cell expressing Ia (immune response gene associated) antigens is required for antigen presentation to proliferating and helper T cells. The experiments proposed herein are designed to determine the requirement for accessory cells in the activation of other T cell subsets, in T-B collaboration and in B cell activation. In each case where an accessory cell can be shown to function, the cells will be characterized with regard to their surface markers, in particular their expression of I region encoded antigens and Fc receptors. With this information, the role of accessory cells in regulation of the immune response will be assessed by the use of purified subpopulations of such cells. The mechanism of antigen processing by Ia ion accessory cells will be explored initially with the Ir gene controlled antigens TNP-(T,G)-A-L and TNP-(H,G)-A-L, for which one cellular site of Ir gene control has been mapped to the accessory cell. Using (responder x non-responder) Fl accessory cells attempts will be made to determine whether or not antigen is presented as a molecular complex with I region gene products.